Solving the Runoff and Development Mystery

An occasional leech comes with the territory when you’re studying
leaching and other aspects of stormwater retention, as does the
occasional incident of equipment tampering. At Ironbound, some curious
soul had pulled the cable from its tube.

The 400 retention ponds of James City County might attract casual
curiosity, as do the hundreds of thousands like them across the
country, but serious study of how well the ponds work has been lacking,
until now.

Hancock, associate professor of geology at the College of William &
Mary, began serious study in the summer of 2004, starting with two
fairly new ponds. Hancock and his Environmental Geology students began
“wandering around” the minor waterways of James City County and York
County, checking on the health of the usually unnamed and often
unnoticed streams that combine to make important watersheds.

Many Ponds, Little Data

“One of the topics in class is how stream systems respond to
urbanization. I recognized that there were a lot of hypothetical ideas
about how streams and watersheds respond to development, but relatively
little data,” Hancock said. “So we started collecting information about
how stream flows vary with different types of development.”

The ponds, one at Mulberry Place and another at Ironbound Village,
were selected for monitoring. Assisted by Marshall Popkin, an
undergraduate who wrote his senior thesis on the ponds, Hancock
installed sensors in each pond that record the water level every 15
minutes and store the data in a minicomputer on site. Each pond also
has an instrument that records rainfall. Every week or two, Hancock or
Popkin or both visit the sites, extract the data and bring it back to
the lab. Back in the lab, they compare actual measured data to the
predicted model from which the pond was designed.

James City County has adopted regulations more stringent than most
localities. Hancock explained that retention ponds built in the county
are required to be designed to detain water from the “one-year, 24-hour
storm” for 24 hours.

The one-year, 24-hour storm event in James City County amounts to 2.8
inches of rain over 24 hours. In other words, the ponds should be able
to accept runoff generated by 2.8 inches of rain falling over the
course of a day on the development they serve—then hold it, releasing
the water slowly over 24 hours. The standard also means that the ponds
can expect to see those 2.8 inches of rain, on average, once a year.

“So we would expect over the course of the year, for instance, to see
only once a flow out of these ponds that would be the equivalent of a
one-year flow,” Hancock said.

The 2.8 inches can be placed in context by comparison with the 1.5
inches of a normal rainy day on the Peninsula, and by the 5 or 6 inches
that came with Hurricane Isabel. “Our gold standard is Hurricane
Floyd,” Hancock said. “19 inches in 24 hours.”

Of course, the ponds are built with rainfall of more than 2.8 inches in
mind. When an Isabel or a Floyd hits—or even just a really heavy
rain—the pond will discharge the excess more quickly. There’s also an
emergency spillway built into the dam for safety.

During the months that Hancock and Popkin have been studying the ponds,
there have been no Isabels or Floyds, or even a day’s rain equaling the
2.8 inches that the ponds were designed to hold. Yet each of the ponds
being monitored performed as if it had been hit by more than 2.8 inches
on multiple occasions.

“In actuality we have seen outflows that have exceeded the one-year
flow many, many times,” Hancock explained. “At one of our sites,
Ironbound Village, we’ve seen the one-year flow ten times. At Mulberry
Place we’ve seen it seven or eight times.”

Hancock acknowledges that the performance of these ponds needs more
study: he intends to add more ponds (and more undergraduate
researchers) to his list. He also will expand the scope of the study to
monitoring stream health below the ponds.

He says that so far, results to date back his initial concern about the
efficacy of ponds in the area. He’s shared his results with James City
County officials and hopes to see his work influence new regulations
governing pond design.

“I’m not entirely sure that there is a problem yet, but if there is a
problem with how these ponds are working, I’d like to do a couple of
things,” he said. “First, figure out what is actually the cause of any
design flaws and work with the county to address it. The other thing
is, if we find that there’s an issue here, it’s going to be something
that’s an issue nationally because these ponds are being designed
similarly everywhere.”